About 15% of the world's available natural gas reserves comprise appreciable amounts of compounds of low calorific value such as nitrogen. The presence of these compounds can be natural or result from the use of an enhanced recovery process. The presence of such compounds, notably in large amounts (with concentrations typically above 4% mol) reduces the calorific value of the gas. The characteristics of the gas are then incompatible with existing commercial transportation and distribution systems specifications since its calorific value can then be below 960 BTU/SCF which corresponds on average to nitrogen contents above 4% by moles.
Less than 10% of the gas fields containing nitrogen in large amounts (more than 4% mol) are currently developed.
Various methods allowing to remove nitrogen are known from the prior art.
It is thus possible to perform cryogenic distillation. Such a process generally leads to high investment costs.
It is also well-known to use adsorption processes for denitrogenation and a certain number of patents refer to them, based either on thermodynamic selectivities or equilibrium selectivities (U.S. Pat. Nos. 4,578,089, 5,174,796 and 5,536,300) or diffusion selectivities (U.S. Pat. Nos. 4,964,889, 4,935,580 and 2,843,219). The latter originate from the difference between the adsorption kinetics of the molecules to be separated, whereas thermodynamic selectivities originate from a difference in the adsorbed quantity at equilibrium.
These processes are characterized by considerable dead volumes since the interstitial volume (volume between the adsorbent particles) commonly represents 30% of the adsorber volume. In the case where adsorbers are used at high pressures, large amounts of the less retained compounds can lodge themselves in these dead volumes and can thus be produced, not during the production phase, but during the regeneration phase, leading to a decrease in the recovery ratio per pass of these compounds.